This invention relates to fluid conductive couplings for fluid transfer conduits and more particularly to such couplings which are flexible and can accomodate, without failure, linear as well as angular displacements between the confronting ends of the conduits coupled thereby.
Flexible fluid conductive couplings are well known, and are employed in a variety of applications where it is desired to connect together conduit sections of a fluid transfer system so as to permit displacement or misalignment of the conduit sections relative to one another. Typical uses are in systems in which alignment of the conduit sections cannot be assured because of considerations of mechanical tolerance and relative motion between the sections (as may be produced by thermal expansion, vibration, system operation, or the like).
A commonly encountered coupling, particularly in systems in which a large torque must be communicated from one conduit section to another (as in well drilling apparatus) or in which the internal fluid pressure is greatly different from the external pressure, is a ball and socket type joint incorporating a distortable combination seal and bearing laminated from alternating layers of relilient and rigid material, such as disclosed in U.S. Pat. Nos. 3,680,895 and 4,068,868. Such joints, however, are dimensioned on the basis of the size of the joined conduit, the radius of the ball and socket being only slightly larger than that of the conduit, and therefore do no necessarily provide for optimum torque coupling through the laminated seal. Further, such joints when used singularly will only accommodate lateral angular misalignment of the joined conduit sections. To accommodate lateral linear motion or misalignment of a pair of conduits as well, a compound coupling comprising a pair of such ball and socket joints separated by a third section of conduit may be employed. Alternatively, both linear and angular lateral motion or misalignment may be accommodated by a coupling comprising a section of flexible conduits.
The compound type of flexible coupling accommodates relative linear motion between the conduit sections it joins together by rotating about an axis normal to the motion. In doing this, the compound coupling produces an adverse couple, i.e., a linear relative motion between its ends normal to both the original linear motion being accommodated and the axis of rotation, and the laminated seals of the ball and socket joints must provide for this adverse coupling to prevent stress on the conduit or joints or parting of the seal. In order to minimize the adversely coupled linear motion, the compound couple should extend between the conduits to be joined in a direction substantially normal to the motion to be accommodated and should have a dimension greater than the motion by as large a factor as possible. These spatial requirements of a compound flexible coupling represent a disadvantage in that they may be in conflict with other requirements of the fluid transfer system (e.g., short pipe run, compact system, etc.).
An additional disadvantage of the compound flexible coupling is in its complexity, in that it requires a pair of ball and socket joints, each having a laminated seal. This not only results in higher fabrication, installation, and maintenance costs, but impacts on the reliability of the coupling as well. Inasmuch as the seal constitutes a source of potential fluid transfer system failure, the use of couplings requiring a multiplicity of distortable seals may not be desirable in critical situations (i.e., situations in which access to the seals for maintenance or replacement is difficult or systems in which loss or contamination of fluid results in excessive danger or damage).
These disadvantages may be overcome, in part, by a coupling incorporating a flexible conduit section bonded securely between the sections of the fluid transfer system to be joined. Such a flexible coupling accommodates relative motion between the conduit sections it joins through the resilient distortion of its walls, and may accommodate, without undue adverse coupling, both linear and rotational motion. Further, such a flexible coupling may avoid the problems of complexity and seal integrity characteristic of the compound ball and socket coupling. However, although potentially shorter than the compound ball and socket coupling (because of its smaller adverse coupling) the extent of the flexible conduit in the direction of fluid flow is still considerable. Further, the flexible conduit is itself a weak link in a fluid transfer system, having a higher susceptibility to failure under extremes of pressure and a smaller torque transmitting capability than does the ball and socket coupling.
In either the compound ball and socket or the flexible conduit type flexible coupling, the component most subject to failure (i.e., the laminated seal or the resilient wall) is not only subject to mechanical loads (e.g., vibration and power transmission, either translational or rotational) and fluid pressure, but also is generally exposed, at least in part, to the fluid stream. This may result in excessive wear of these critical componets in cases where the fluid stream contains particulate matter.